1. Field of the Invention
The present invention relates to a control device for a machine tool which performs oscillation cutting.
2. Description of the Related Art
When swarf is continuously generated during the machining of a workpiece using the cutting tool of a machine tool, the swarf may become entangled with the cutting tool. In such a case, it is necessary to stop the machine tool in order to remove the swarf from the cutting tool, which is time consuming and reduces production efficiency. Further, there is a risk that the workpiece may be damaged by the swarf, reducing the quality of the workpiece.
In order to avoid such drawbacks, oscillation cutting, in which swarf is shredded by oscillating the cutting tool relative to the workpiece in the machining feed direction, is known (refer to, for example, Japanese Patent No. 5033929, Japanese Patent No. 5139592, or Japanese Patent No. 5599523).
Regarding the oscillation cutting described above, techniques for avoiding the problem that the cutting tool cuts beyond a predetermined machining stop position by reducing the oscillation amplitude of the cutting tool as the cutting tool approaches the predetermined machining stop position on the workpiece in the machining feed direction have been proposed (refer to, for example WO 2016/047485). Hereinafter, such a problem will be referred to as overshoot.
Furthermore, in machine tools, learning control has been proposed to increase the positional accuracy of driven objects such as cutting tools or workpieces driven by servo motors (refer to, for example, FIG. 6 of Japanese Unexamined Patent Publication (Kokai) No. 2006-172149). Learning control is a control in which, with respect to a signal to command a driven body to perform the same pattern of operations at a predetermined period, a correction amount necessary to correct position deviation, which is the difference between the position command value and the position feedback value, and the correction amount which has been obtained one period prior is applied to the position deviation. By repeatedly performing this learning control for the same pattern of the periodic operations, a correction amount for converging the position deviation to zero can be obtained.
In a machine tool that performs the above-mentioned oscillation cutting, if there is backlash in the drive mechanism part of the cutting tool or workpiece or when the rigidity of the drive mechanism part is low, vibration is generated if the control gain is set high in order to improve the responsiveness of the servo, such that the position accuracy of tool or workpiece may not be stable. In such a case, even though it is difficult to improve the responsiveness of the servo, if learning control is applied to the oscillation cutting, it is possible to achieve high-precision control for the periodic motion commands that relatively oscillate the tool or workpiece in the machining feed direction.